Nonanomeric carbons

These observations have several implications for studies of glycoproteins (i) the resonances of anomeric carbon atoms that are involved in N-glycosylic linkages will be rather difficult to identify, because of their proximity to nonanomeric-carbon resonances and (ii) due to the proximity of the chemical shifts of C-l and C-5 (they are only 0.6 p.p.m. apart for / -d-G1cNAc — Asn), it may be difficult, but not impossible, to ascertain whether N-glycosylic linkages exist in the glycoprotein (see later). 

This pattern of chemical-shift nonequivalence is also manifested by the nonanomeric carbon atoms. 

The sugar molecule can be considered as a playground for exploratory organic s)mthesis, and many functional group manipulations can be carried out on the nonanomeric carbons of carbo-... 

The substitution reactions at the anomeric carbon usually proceed easily and via a SnI mechanism in most cases. However, it is not the case in the nucleophilic substitutions at nonanomeric sites. Because of the presence of vicinal electron-withdrawing substituents (OR or NHR) which strongly destabilize the intermediate carbocations, Sn2 displacement reactions instead of Sn 1 reactions are favored (O Scheme 1). 

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